Print this model and then buy a glass lens for $4.50 from Surplus Shed
http://www.surplusshed.com/. Their part number is L4581 (NOTE - THIS IS A NEW UPDATED LENS CHOICE. The previous L4471 is now too small at 11mm diameter. This 3D print requires a 12mm diameter lens). The search function on the Surplus Shed site won't search part numbers. To find L4581, use the "Lens Finder" on their homepage.
In "Lens Finder" type:
Lens Type: PCX, Plano Convex
Diameter: 12mm to 12mm
Focal length: 39mm to 39mm
That will get you to L4581. If you want other magnifications, search for other 12mm diameter lenses. A longer/shorter focal length means less/more magnification. Feel free to experiment.
The lens fits in a recess in the Shapeways model and is held in place with a drop of Superglue. If you get any glue on the lens, it can be cleaned off with nail polish remover or acetone.
NEW EASIER ASSEMBLY INSTRUCTIONS !
The lens is plano-convex (one side is flat). When inserted in the model, the flat side of the lens should face the phone. Press the lens into the opening before applying any glue. Make sure it is seated all the way against the support ring at the bottom of the hole. Again, do this first WITHOUT glue. Once you are sure the lens is properly seated, squeeze one TINY drop of superglue into the seam between the lens and the plastic holder at two spots on opposite sides of the lens circle (from the front). Two drops should be enough. The drop should wick around the edge the lens by itself so you won't need to spread it around. Let the assembly sit for 5 minutes before doing any lens cleaning.